1. Field of the Invention
The present invention relates to a capacitor charging circuit, and in particular to technology for monitoring output voltage of the capacitor charging circuit.
2. Description of the Related Art
To generate voltage that is higher than input voltage, voltage boosting capacitor charging circuits are widely used in various electronic devices. Such voltage boosting capacitor charging circuits are provided with a switching transistor, and an inductor or a transformer, and by turning the switching transistor ON and OFF in a time-division manner, back electromotive force is generated in the inductor or the transformer, and input voltage is boosted and outputted.
When the switching transistor is turned OFF, a current flows on a primary side of the transformer and energy is stored in the transformer. When the switching transistor is turned OFF, energy stored in the transformer on a secondary side of the transformer is transferred to an output capacitor, via a rectifier diode, as a charging current, and the output voltage rises. By a switching operation of the switching transistor, output voltage of the capacitor charging circuit increases.
Patent Document 1: Japanese Patent Application, Laid-Open No. 2004-201474
Patent Document 2: Japanese Patent Application, Laid-Open No. 2005-73483
Patent Document 3: Japanese Patent Application, Laid-Open No. 2007-165002
Patent Document 4: Japanese Patent Application, Laid-Open No. 2007-166786
Unlike a switching regulator or the like, when the capacitor charging circuit performs a switching operation of the switching transistor, the output voltage continues to increase. Therefore, the capacitor charging circuit has to monitor its output voltage, stop switching when a certain target value is reached, and perform overvoltage protection when a certain threshold is reached.
Here, when the output voltage is monitored, consideration is given to cases in which the output voltage is indirectly monitored using voltage generated by the transformer, rather than directly monitored by resistance voltage division or the like. The voltage that is to be monitored in such cases is synchronous with ON and OFF states of the switching transistor, and alternates between a value corresponding to the output voltage and a value with no relation thereto. Furthermore, since ringing occurs due to an effect of a resonant circuit including a transformer, accurate voltage detection is difficult.